In late winter 2012, attention was drawn to a new insect/pathogen complex in Southern California as a result of damage to several backyard avocado trees in residential neighborhoods and a commercial avocado grove in Los Angeles County. The insect/pathogen complex is now known to be found over a wide area including most of Los Angeles County, parts of Orange and San Bernardino Counties; and, in late 2013, an outbreak in San Diego County.[i] [see map]. This outbreak is 60 miles from the nearest outbreak, and indications are that the presence in San Diego is the result of a separate introduction, not spread from Los Angeles. [ii]

In 2014, the disease was detected for the first time in a commercial avocado nursery. A “grower alert” suggested steps growers should take to avoid spreading the disease. The focus was on disposal of wood from infested trees – including as firewood, pruning debris, or chips; and on ensuring that twigs and leaves are not present in bins used to ship the fruit. Growers are also advised to inspect not just their avocado trees but other hosts (see below) growing near their orchards. [iii]

The disease is caused by a new, as yet unnamed Fusarium fungus species that forms a symbiotic relationship with a beetle in the Euwallacea genus, which serves as the vector. The beetle transports the fungus; in turn, the beetle larvae live in galleries within the tree and feed on the fungus. The beetle has been named the polyphagous shot hole borer (PSHB). This beetle is morphologically indistinguishable from the tea shot hole borer, Euwallacea fornicatus, an ambrosia beetle from Asia. However, scientists suspect that the California beetle is a new species, based on the large differences in DNA sequence between the beetle invading California and beetles from tea plantations in Asia. The beetle discovered in California is smaller than a sesame seed (about 0.1 inch in length). The identical new beetle species has been causing damage to commercial avocado orchards in Israel since 2009. [iv]

The polyphagous shot hole borer has been in California since at least 2003, when it was found on some black locust (Robinia pseudoacacia). [v]

Studies at the Los Angeles County Arboretum and Huntington Library, Arts Collections, and Botanical Gardens have identified 207 species of tree, shrub, or vine that are attacked by the beetle; these plants are in 58 plant families from every continent except Antarctica. The Fusarium fungus was detected in 54% of these trees – 113 species. Among the trees attacked by the beetle are 11 species native to the southern California; 13 agriculturally important trees; and 53 species widely used in urban plantings. Analysis of a typical metropolitan area’s tree survey indicates that these latter constitute more than half of all trees planted in urban areas of southern California. [vi]
The costs to cities and homeowners of dealing with the disease could be substantial; among hosts are such large trees as coast live oak and several Eucalyptus.

Some California ecosystems are at particular risk because they are dominated by susceptible tree or shrub species. These vulnerable ecosystems are mixed evergreen forests, oak woodlands, foothill woodlands, & riparian habitats. [vii]

Host trees native to southern California include (those marked by an asterisk are reproductive hosts):[viii]

*Acer macrophyllum - Big leaf maple

*Acer negundo - Box elder

*Quercus agrifolia - Coast live oak

Quercus chrysolepis - Canyon live oak

* Quercus engelmannii - Engelmann Oak

Quercus lobata - Valley oak

Umbellularia californica - California bay laurel

Fraxinus velutina – velvet ash

*Platanus racemosa - California sycamore

Rhamnus californica - Coffee berry

Many more host trees are native to other parts of the United States:

Acer × freemanii - Freeman maple

Washingtonia filifera - Desert fan palm

Alnus rhombifolia - white alder

Catalpa speciosa - Northern catalpa

Juniperus virginiana - Eastern red-cedar

Jatropha cf. cinerea - Limberbush

Erythrina coralloides - Flame coral tree

Quercus alba - White oak

Quercus macrocarpa - Bur oak

Quercus rubra - Northern red oak

Quercus virginiana - Southern live oak

Liquidambar styraciflua - American sweetgum

Carya illinoinensis - Hickory

Juglans nigra - Eastern black walnut

Magnolia grandiflora - Southern magnolia

Platanus occidentalis - American sycamore

Platanus wrightii - Arizona sycamore

Tilia americana - Basswood

Ulmus alata - Winged elm

Ulmus americana - American elm

Only one – boxelder – is known to be a reproductive host. But the list of reproductive hosts might be incomplete, since researchers were unable to do destructive sampling of all suspect trees growing in the botanical gardens. [ix]

Many hosts are common in horticulture or urban plantings. These hosts would not only maintain and spread the infestation locally; they might also facilitate long-distance spread via the trade (although the insect attacks only trees or shrubs of a certain size, [x] and only two are listed as reproductive hosts). Horticulturally important hosts include, in addition to several of the species already listed:

*Acer palmatum – Japanese maple

Ilex aquifolium – English holly

Betula pendula – silver birch

Wisteria sinensis & W. floribunda – wisteria

Fagus sylvatica - European beech

Quercus ruber – English oak

Ficus benjamina & F. maxima – figs

Malus floribunda – Japanese crab apple

Prunus serrulata – Japanese cherry

Salix – willow

Several Koelreuteria – rain trees

*Several Camellia

The hosts important to commercial agriculture include avocado eastern mulberry, olive, macadamia, Turkish hazelnut, loquat, peach, grapevine, and citrus. Citrus appears to be attacked uncommonly. Of these, avocado is the only confirmed reproductive host. Beetle attack symptoms have been detected on cassava, Japanese persimmon, Japanese crabapple, and eastern black walnut, but no Fusarium fungus was present. [xi]

A few hosts are common invasive plants that could help maintain or spread the infestation, in California or elswhere:

* Ricinus communis - Castor bean

Schinus terebinthifolius - Brazilian pepper tree

Sapium sebiferum - Chinese tallow tree

Ailanthus altissima - Tree of heaven

Finally, many more hosts are native to forests of Mexico or farther south in the Americas.

When beetles attack a tree, there are several potential outcomes: [xii]

1. The beetle is repelled and causes no infection. This has been observed in 20 species of trees. Investigators are trying to figure out what features of the tree might repel the beetle.

2. The beetle drills into the tree and transmits the fungus, but doesn’t produce offspring. This has been observed in over 50% of the tree species attacked. Scientists don’t know the final outcome of this interaction. If the beetle has penetrated to the xylem, this could cause dieback of branches. Damage could also make the tree more prone to attack from other pest species.

3. The beetle drills into the tree, fungus infects the tree, and the beetle produces offspring in the tree. This has been seen in about 8% of the tree species attacked, and these species are considered reproductive hosts of PSHB. Some trees seem to suffer mild symptoms like branch die-back, while others are killed outright.

The 24 species listed by Dr. Eskalen [xiii] as reproductive hosts include

Avocado, maples, coast live oak, and sweetgum are in very different botanical families (Laureaceae, Aceracae, Fagaceae, Hamamelidaceae) -- so the host range could be quite large. Thirty three families contain both species that are hosts of the beetle and/or the fungus; and species that apparently do not host them. [xiv]

Given the large number of tree types that can be infested by the beetle, and the growing list of species that support beetle reproduction, it appears highly likely that the disease complex could be transported in firewood. Firewood suppliers and customers, as well as regulatory agencies should take action to ensure that this does not happen.

Symptoms of PSHB attack and fungus infection differ among tree species. For illustrations of the symptoms on various species, visit www.eskalenlab.ucr.edu